I might have misunderstood the way CRISPR-Cas9 experiment works, but what I understood is that, alongside other things, a gRNA is used to "guide" the Cas9-protein into:

• Cut at the right position
• Make the cell build a new DNA sequence at the position

I found a lot of examples of tools, such as CRISPRSeek, that are used to select gRNAs that will correctly guide the Cas-9 protein in accordance with the purpose of the experiment.

Now what I don't really understand is where does the gRNA comes from?

For an experiment, it is apparently bought to some companies. But how does they get it? My hypothesis:

• Is it synthetized ? If yes, how?
• Is it "taken" from organisms (wikipedia about gRNA mentions mitochondries), and if yes how?